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The effect of buoyancy on fetal/larval cardiac output and the role of atrial natriuretic peptide in compensation during parturition

The effect of buoyancy on fetal/larval cardiac output and the role of atrial natriuretic peptide in compensation during parturition

Author

Snyder, Brad E.

Keywords

University of New Orleans. Honors thesis. 1997.; University of New Orleans. Dept. of Biological Sciences.

Abstract/Introduction

Changes in posture have dramatic effects on the cardiovascular system. Blood collects in the lower extremities due to gravity when we stand, creating a substantial increase in hydrostatic pressure. Collection of blood creates a reservoir which can be compressed to increase the pressure in the central venous system (i.e. the large intrathoracic veins that return to the heart). As a result, the pooling of blood in the lower extremities affects the return of blood to the heart, known as venous return to the heart. This venous return to the heart is the major component of cardiac output. Cardiac output is defined as the volume of blood pumped from one ventricle per unit of time. Therefore, the more blood that returns to the heart in one minute the higher the cardiac output will be. Other factors (e.g., ventricular distensibility and heart rate) also determine cardiac output; however, venous return to the heart is the most critical variable because it is influenced by many factors at once. It was noted in 1992 that a space crew's cardiac outputs increased by 50% when they left the Earth's gravitational field (Hoppe 1992). As a result, I reason that cardiac output may indeed increase as a result of buoyancy. When we are buoyant, blood reservoirs dissipate and central venous pressure decreases, while cardiac output increases. I hypothesized that these observations were a result of increased venous return to the heart. Here, I demonstrate that buoyancy can reduce resistance of blood flow to the heart. When there are no blood reservoirs, blood is able to flow through the residual reservoirs with great ease and. consequently, blood can flow to the heart much more readily.
The results of this experiment have led me to consider the cardiovascular condition of embryos/larvae. That is, what is their cardiovascular physiology while they are floating in the amnion? To determine what relationship buoyancy has on cardiac output, data for amphibian larvae's and chicken embryo's cardiac output were matched with the buoyancy of human fetuses throughout development. After the fetuses were normalized with an appropriate embryo/larvae, a correlation confirmed that there is indeed a significant relationship between cardiac output and buoyancy in both cases.
Finally, the recently discovered atrial natriuretic peptide also proved to be an important player in the relationship amoung cardiac output, buoyancy, and embryos/larvae. This natriuretic, released in response to increased stretch of the atria, is believed to play a role in volume regulation by causing the loss of sodium and the concomitant loss of water. However, it had been verified that this peptide is not a functional natriuretic at physiological dosages. Furthermore, it is shown that high concentrations can be found in animals that are submerged in water. This suggested that as cardiac output is increased due to buoyancy, atrial natriuretic peptide is released to reduce the cardiac output. Furthermore, it has been demonstrated that fetuses have very high concentrations of atrial natriuretic peptide relative to maternal concentrations. This observation, as well as many others (e.g. an increase in atrial natriuretic peptide concentration as the day of parturition gets closer) suggests that fetuses use this peptide as a compensatory mechanism for parturition. That is, they keep their blood volume low until they are born so that the resulting increase in hydrostatic pressure due to gravity after parturition is not devastating.
All of these collected data demonstrate a cardiovascular adaptation to terrestrial life. Our increase in cardiac output due to buoyancy may not be that convincing; however, the mechanisms established to insure that the increased cardiac output is stabilized or reduced is very persuading, and the fact that this mechanism is present during prenatal stages is convincing that this relationship between cardiac output and buoyancy is as old as amphibians.

Access

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Degree

B.S.

Degree Program

Biological Sciences

Department

Department of Biological Sciences

Thesis Advisor

Stevenson, Michael

Advisory Committee

Johnson, Steven; Mooney, Michael E.

Date Degree Awarded

1997;

Rights

The University of New Orleans and its agents retain the non-exclusive license to archive and make accessible this thesis in whole or in part in all forms of media, now or hereafter known. The author retains all other ownership rights to the copyright of the thesis.

Notes

vii, 53 leaves : ill. ; 29 cm.; typescript; "An honors thesis ... in partial fulfillment of the requirements for the degree of Bachelor of Science, with University Honors and Honors in Biological Sciences"; Thesis (B.S.)--University of New Orleans, 1997; Includes bibliographical references (leaves 50-53); May 1997; approved May 8, 1997.